Extensions Docs Server: Fix headings with no IDs

chrome/common/extensions/docs/templates/articles/background_pages.html

 <h1>Background Pages</h1>
 
 
 <p id="eventPageWarning" class="warning">
   <em>Caution:</em> Consider using event pages instead.
   <a href="event_pages.html">Learn more</a>.
 </p>
 
 <p>
 A common need for extensions is to have
@@ skipping 65 matching lines @@
 }</pre>
 
 <p>
 If you need the browser to start up early&mdash;so
 you can display notifications, for example&mdash;then
 you might also want to specify the
 <a href="manifest.html#permissions">"background" permission</a>.
 </p>
 
 
-<h2>Details</h2>
+<h2 id="details">Details</h2>
 
 <p>
 You can communicate between your various pages using direct script calls,
 similar to how frames can communicate.
 The <a href="extension.html#method-getViews"><code>chrome.extension.getViews()</code></a> method
 returns a list of window objects
 for every active page belonging to your extension,
 and the
 <a href="extension.html#method-getBackgroundPage"><code>chrome.extension.getBackgroundPage()</code></a> method
 returns the background page.
@@ skipping 53 matching lines @@
 
   &lt;body>
     &lt;p>
     Image here:
     &lt;/p>
 
     &lt;img id="target" src="white.png" width="640" height="480">
 
   &lt;/body>
 &lt;/html>
-</pre>
+</pre>